1. Field of the Invention
The present invention relates to an image forming apparatus and a method for controlling the image forming apparatus.
2. Description of the Related Art
Recently, computerization of information has been promoted. In the computerization, image processing apparatuses, such as printers and facsimiles used for outputting computerized information and scanners used for computerizing documents, have become indispensable. Such image processing apparatuses are mostly structured as multifunction peripherals that can be used as printers, facsimiles, scanners, and copying machines with image capturing, image forming, and communications functions, for example, provided therein.
Among the image processing apparatuses, electrophotography image forming apparatuses have been widely used as the image forming apparatuses used for outputting computerized documents. In an example of such electrophotography image forming apparatuses, a photosensitive element is exposed to light so as to generate a static latent image, and then the static latent image is developed by using a developer such as toner to generate a toner image, and lastly paper output is carried out by transferring the toner image onto a sheet.
In such electrophotography image forming apparatus, operational timing of exposing the photosensitive element to light to generate a static latent image and operational timing of sheet conveyance are adjusted to be synchronized so as to generate an image in a desired area on a sheet. In an image forming apparatus that forms a color image by using a plurality of photosensitive elements, which is known as a tandem type image forming apparatus, exposure timing among color photosensitive elements is adjusted so that images developed on the respective color photosensitive elements are accurately overlapped. Hereinafter, these adjustment processes are collectively referred to as positional deviation correction.
In the positional deviation correction, a timing detection pattern serving as an adjustment image is formed in the same operation as normal operation of photosensitive element exposure and static latent image development, and then the pattern is read by a reflective light sensor. A period from when photosensitive element exposure starts to when the timing detection pattern is read is counted. The counted period is compared with a predetermined reference value, and adjustment processing is carried out based on the difference between the counted period and the reference value.
The timing detection pattern is formed on an intermediate transfer belt in an image forming apparatus employing an intermediate transfer belt system in which a toner image is transferred onto the intermediate transfer belt from a photosensitive element, and thereafter transferred onto a sheet. The timing detection pattern is formed on a conveying belt conveying a sheet in an image forming apparatus employing a direct transfer system in which a toner image is directly transferred onto the sheet from a photosensitive element. In an image forming apparatus having no intermediate belt or no conveying belt, i.e., an image forming apparatus employing a beltless system, a method is proposed in which a position adjustment pattern is printed on a conveyed sheet. For example, refer to Japanese Patent Application Laid-open No. 2008-299311.
In positional deviation correction, a sensor that reads a timing detection pattern irradiates a surface of a sheet on which the timing detection pattern is formed, and receives reflected light from the surface of the sheet so as to detect the pattern based on a voltage of a signal obtained according to a received light amount. The voltage obtained according to reflected light shows a maximum in reflected light from a white region in which no pattern is formed. In a region in which the pattern is formed, a light amount of reflected light decreases because the pattern absorbs the light, and thus the voltage lowers. Accordingly, the pattern can be detected by detecting a change from a reference voltage that is set to a voltage obtained based on the reflected light from the region in which no pattern is formed.
In the positional deviation correction, a driving voltage or a driving current that drives a light source included in the sensor is adjusted according to a fluctuation in the light source or a fluctuation in a gloss level of the white region, in order to obtain a constant reference voltage. The adjustment is carried out to prevent the white region from being wrongly detected as the region in which the pattern is formed due to weak reflected light from the irradiated white region when the white region has a low gloss level or the light source has a low light amount.
In an image forming apparatus including an intermediate transfer belt or a conveying belt, the intermediate transfer belt or the conveying belt is used as the white region. In other words, a driving voltage or a driving current is adjusted in such a manner that a voltage obtained according to reflected light from a surface of the intermediate transfer belt or the conveying belt becomes a predetermined value. The adjustment of a driving voltage or current that drives the light source is carried out mainly to address a fluctuation in a gloss level of the white region, i.e., the intermediate transfer belt or the conveying belt, caused by stains thereon.
In the beltless system, however, the above-described adjustment of a driving voltage or current that drives the light source is not carried out because the white region corresponds to a sheet newly conveyed, and thus it is not necessary to take stains into consideration unlike the case with the other systems (intermediate transfer system and direct transfer system).
Recently, types of sheets, including recycled paper and photo paper, which are used for image forming output in addition to regular paper have increased. Even in the beltless system, a pattern may be wrongly detected in positional deviation correction as described above, because different types of sheets have different gloss levels. Accordingly, the adjustment of a driving voltage or current of a light source as described above is also desired in the beltless system.
As describe above, in the beltless system, positional deviation correction is carried out by using a positional deviation correction pattern formed on a conveyed sheet. In sheet conveyance of the beltless system, a sheet is more likely to be undulated than a sheet conveyed on a conveying belt by being sucked to the belt. The undulation of a sheet causes a distance between a light source and a reflecting surface to fluctuate, resulting in intensity of detected reflected light being fluctuated. The coincidental occurrence of sheet undulation and a gloss level fluctuation of a sheet surface further increases a likelihood that a pattern is wrongly detected in the beltless system.